If I understand it correctly, having money in these types of offset accounts doesn't earn you interest. By reducing the balance of your mortgage, albeit temporarily, it reduces the interest you pay on your mortgage. The money saved is not therefore subject to DIRT. I always thought this was one of the main selling points of these types of mortgage account.
To make a like-for-like comparison, therefore, you need to compare the offset account rate with the after-tax rate of the savings account.